We are trying to make large area substrate gated thin film transistor structures by spin coating with a polymer. We have tried PMMA, but get low reliability on 5mm x 5mm devices. We get better results by spin coating multiple PMMA layers, but the yield is still low on 1cm x 1cm devices. The structure needs to be transparent.
Dear Sir,
There are a lot of factors which control the spinning process in the lithography process.
1. The recipe has to be selected very carefully ( I mean time of spinning and rate of spinning). These two should be well calibrated.
2. The spinning process also depends on the type of process (lift-off or etching) you wish to do after spinning the resist on the substrate.
3. It also depends on the feature size required on the device. How much feature size you require?
Have you tried with PPR resist?
Did you try changing the time of spinning and rate of spinning?
Spin-coating on larger substrates can sometimes lead to better results since the edge effects are not so significant. I suggest PMMA of enough high concentration (1-3 w%) and enough high spin rate (higher spin rate = higher homogeneity). To avoid short circuit it is better to have lower thickness of bottom patterned electrode because spin-coating has always problems with edges. The choice of solvent is also important due to wettability of the substrate (i.e. contact angle).
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